I have a question that is of no consequence and will not change in any way my photo skills ( it'll take more than that.. ) but nevertheless it occupies my mind...

First , to provide some background to my (mis)understanding , I understand a lenses aperture expressed by it's f-stop to be something like focal length / aperture . OK all good.

But , when a zoom is constant aperture , the focal length changes throughout the zoom range ... So how does the aperture change to maintain the F number ? , I.e after using a 70-200 2.8 ,@ 200mm does the aperture max size suddenly become a lot smaller when moving towards the 70mm end ??.

"An f-stop is the ratio between the focal length of the lens and the *apparent* size of the lens opening as viewed through the front. It must take into account the magnification factor of all lens elements in front of the diaphragm, because it is the size of the opening that the light "sees" as it passes through the lens, not the actual physical diameter of the diaphragm opening. It is this fact that allows companies to make constant aperture zoom lenses which maintain a constant f-stop when the focal length changes, because such lenses are designed so that the magnification factor (diopter value) of all elements in front of the diaphragm changes as focal length is changed to hold the aperture value constant."

That make sense . i like your explanation of the aperture as " *apparent* size of the lens opening as viewed through the front" , tech resources on the net talk about entrance pupils and the like, which is all well and good , but every description of these terms has been far too technical for my limited optical vocabulary!.

Here's what Chuck Westfall, Canon's technical guru/mouthpiece, had to say on the matter:

For a simple lens, the definition of f/stop is focal length divided by the diameter of the front element. But SLR zoom lenses are far from simple, and there are many different types, such as wide-to-wide, wide-to-telephoto, and telephoto-to-telephoto. In all of these lenses, it's the apparent size of the aperture, i.e., the "virtual aperture" that counts, not the size of the physical aperture. In the case of a traditional telephoto zoom lens like the EF70-200/2.8L, you can see the virtual aperture change if you look through the front of the lens while you're zooming it. It increases in diameter as you zoom towards 200mm, and decreases as you zoom towards 70mm. But the size of the virtual aperture is directly proportional to the focal length setting, resulting in an effective aperture that's constant at all focal lengths. So an EF70-200/2.8L should really be thought of as a 70mm f/2.8 lens with a sort of "zoom teleconverter" in front of its iris diaphragm.

Wide-to-wide lenses like the EF16-35mm f/2.8L II are essentially inverted telephoto zooms, so they function basically the same as the EF70-200/2.8L except that the "zoom teleconverter" is behind the iris diaphragm. Wide-to-tele zoom lenses like the EF24-70mm f/2.8L achieve their constant aperture differently than either the wide-to-wide or tele-to-tele designs; in this case the iris diaphragm is "cammed" so that it changes its size as the lens is zoomed. You can see this if you look through the lens off the camera while you're zooming it. If the diaphragm was not cammed, the 24-70/2.8L would be revealed as a variable-aperture zoom with a maximum aperture larger than f/2.8 at all focal lengths under 70mm.

Wide-to-tele zoom lenses like the EF24-70mm f/2.8L achieve their constant aperture differently than either the wide-to-wide or tele-to-tele designs; in this case the iris diaphragm is "cammed" so that it changes its size as the lens is zoomed. You can see this if you look through the lens off the camera while you're zooming it. If the diaphragm was not cammed, the 24-70/2.8L would be revealed as a variable-aperture zoom with a maximum aperture larger than f/2.8 at all focal lengths under 70mm.

So this is the thing that I've always wondered, is why do they do that?Sigma and/or Tamron have f/2.8-f/4.0 vari-aperture zooms in the 24-100 ranges, which is basically them taking a f/4 constant aperture lens and removing the f/4 restriction on the wide end so that it comes out at f/2.8.

So if we take the EF 24-70 f/2.8, Chuck's just said that we could take away that cam and make it something like a 24-70 f/2.0-2.8.Well, why don't they?Is it just the 'prestige' that constant-aperture is better than variable? (even though it only really affects you if you're shooting wide-open in M or using flash). (and if you've got a f/2.0-2.8 vari-aperture zoom and using M or flash, you can just set the aperture to f/2.8, when you zoom out it will stay at f/2.Would the IQ suffer that badly? (given how well the 24-70 II performs, I don't think it would do that much worse at 24mm f/2.0).Do they want to protect sales of the 24mm f/1.4 L? ([/cynical]).The only real downside I can see is vignetting at the widest end, it's already 1.8 stops at 24mm f/2.8, at f/2.0 it might be as bad as the 15-85 @ 15mm. But jam it on a low-noise body like 5D3 or 1DX or even 6D, and you can use PIC in DPP and fix it right up.I know it'll never happen, but it would be a nice thought...